FAQ 5.1 How do I flush/unbuffer an output filehandle? Why must I do this?

Discussion in 'Perl Misc' started by PerlFAQ Server, Apr 2, 2011.

  1. This is an excerpt from the latest version perlfaq5.pod, which
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    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .


    5.1: How do I flush/unbuffer an output filehandle? Why must I do this?

    (contributed by brian d foy)

    You might like to read Mark Jason Dominus's "Suffering From Buffering"
    at http://perl.plover.com/FAQs/Buffering.html .

    Perl normally buffers output so it doesn't make a system call for every
    bit of output. By saving up output, it makes fewer expensive system
    calls. For instance, in this little bit of code, you want to print a dot
    to the screen for every line you process to watch the progress of your
    program. Instead of seeing a dot for every line, Perl buffers the output
    and you have a long wait before you see a row of 50 dots all at once:

    # long wait, then row of dots all at once
    while( <> ) {
    print ".";
    print "\n" unless ++$count % 50;

    #... expensive line processing operations

    To get around this, you have to unbuffer the output filehandle, in this
    case, "STDOUT". You can set the special variable $| to a true value
    (mnemonic: making your filehandles "piping hot"):


    # dot shown immediately
    while( <> ) {
    print ".";
    print "\n" unless ++$count % 50;

    #... expensive line processing operations

    The $| is one of the per-filehandle special variables, so each
    filehandle has its own copy of its value. If you want to merge standard
    output and standard error for instance, you have to unbuffer each
    (although STDERR might be unbuffered by default):

    my $previous_default = select(STDOUT); # save previous default
    $|++; # autoflush STDOUT
    $|++; # autoflush STDERR, to be sure
    select($previous_default); # restore previous default

    # now should alternate . and +
    while( 1 )
    sleep 1;
    print STDOUT ".";
    print STDERR "+";
    print STDOUT "\n" unless ++$count % 25;

    Besides the $| special variable, you can use "binmode" to give your
    filehandle a ":unix" layer, which is unbuffered:

    binmode( STDOUT, ":unix" );

    while( 1 ) {
    sleep 1;
    print ".";
    print "\n" unless ++$count % 50;

    For more information on output layers, see the entries for "binmode" and
    "open" in perlfunc, and the "PerlIO" module documentation.

    If you are using "IO::Handle" or one of its subclasses, you can call the
    "autoflush" method to change the settings of the filehandle:

    use IO::Handle;
    open my( $io_fh ), ">", "output.txt";

    The "IO::Handle" objects also have a "flush" method. You can flush the
    buffer any time you want without auto-buffering



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    so please include as much information as possible and relevant in any
    corrections. The perlfaq-workers also don't have access to every
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